James Harrison, Clay Matthews, and Julius Peppers will be interviewed next month by NFL officials in connection to a media report that linked them to the use of performance-enhancing drugs.

A letter from NFL executive Adolpho Birch that was obtained Friday by The Associated Press says Green Bay linebackers Matthews and Peppers and Pittsburgh linebacker Harrison will be interviewed when training camps open. The Packers begin practicing on July 26, the Steelers on July 29.

The letter also mentions defensive lineman Mike Neal, a free agent who will be interviewed. It does not mention the now-retired Peyton Manning, who also was cited in Al-Jazeera's doping report in December.

But USA Today, citing an unnamed source, reported that an investigation into Manning's possible involvement also is progressing.

The NFL first notified Matthews, Peppers, Harrison and Neal about its investigation into the report on Jan. 11. That investigation has proceeded, but Birch wrote that the NFL Players Association hasn't responded to "multiple requests" to schedule the interviews, which would be conducted with a union representative present.

Al-Jazeera America reported allegations by Charlie Sly, who worked as an intern at an anti-aging clinic. But Sly later recanted his claims.

In his letter, Birch wrote that "the players were further advised that, with their full and timely cooperation, the investigation would be conducted expeditiously and with minimal disruption."

"While the investigation has proceeded," he continued, "we have yet to interview the players. We have attempted since early April to work through the NFLPA to schedule them, but despite multiple requests the NFLPA has failed to respond, except to seek reconsideration of the basis for the investigation. This continuing delay and avoidance has obstructed our ability to conduct and conclude the investigation."

The NFLPA said Friday it had contacted Birch about the nature of the investigation.

"The NFLPA represents its players in conjunction with any investigation by their employer," the union said in a statement. "On behalf of its players, the NFLPA made numerous inquiries of the NFL through Adolpho Birch asking whether the basis of the NFL's investigation of players is entirely predicated upon recanted statements to a person used by Al Jazeera to make secret recordings. The NFL has not provided any other evidence, nor has it informed the NFLPA or players that any such evidence exists.

"As of today, the only additional response from the NFL has been to demand interviews, and apparently to provide correspondence to the media. The NFLPA will continue to represent and advise its players of their rights, and to communicate the players' decisions to the NFL."